Cermet electrodes containing platinum for the electrochemical reduction of oxygen

ABSTRACT

Platinum metal cermet electrodes are described on which the electrochemical reduction of oxygen is made easier compared to known platinum metal cermet electrodes. The ceramic component of a platinum metal cermet electrode contains stabilized ZrO 2  as its main constituent, the composition of the electrode being directed toward reducing its polarization resistance.

This application is a 371 of PCT/DE01/02608, filed on Jul. 12, 2001,which claims priority to German application 100 33 906.9, filed on Jul.12, 2000.

BACKGROUND INFORMATION

The invention relates to platinum metal cermet electrodes for theelectrochemical reduction of O₂, the ceramic component of which containsstabilized ZrO₂ as its main constituent.

The electrochemical reduction of oxygen plays a role in gas sensors forthe analysis of gas mixtures containing O₂. Such gas sensors aredescribed, for example, in German Patent Application 39 34 586 A1,German Patent Application 42 40 267 A1 and German Patent Application 19833 087 A1. The platinum metal cermet electrodes used are printed onsubstrates of Y-stabilized ZrO₂. The platinum metal cermet electrodesnormally have a ceramic component of 40 vol.-% (the supportingstructure) and a 60 vol.-% metallic component. The supporting structureis primarily made up of Y-stabilized ZrO₂ which contains 10 wt.-% Y₂O₃with respect to the ZrO₂ component in terms of order of magnitude. Theceramic addition should ensure a certain porosity of the electrodematerial and it should also extend the electrochemically effective threephase limit of the actual contact surface between substrate andelectrode based on the ionic conductivity of the supporting structureinto areas of the electrode. The O₂ pump flows obtained with the knownelectrodes are still not satisfactory.

THE INVENTION AND ITS ADVANTAGES

The object of the invention is to specify platinum metal cermetelectrodes on which the electrochemical oxygen reduction is made easiercompared to known platinum metal cermet electrodes.

This objective is achieved with platinum metal cermet electrodes of thetype mentioned above having the features of the characterizing part ofClaim 1 The polarization resistance will be defined further below.

According to an advantageous embodiment of the invention, thepolarization resistance of the electrode is reduced by replacing betweenapproximately 0.01 and approximately 1 wt.-% of the stabilized ZrO₂ withstabilized CeO₂. The substitution of a small quantity of the ZrO₂ withCeO₂ makes it possible to attain a considerable reduction of thepolarization resistance and accordingly higher O₂ reduction flows thanwhen standard electrodes are used.

It is even more advantageous to replace between approximately 0.02 andapproximately 0.1 wt.-% of stabilized ZrO₂ with stabilized CeO₂ sincethe polarization resistance is then particularly low.

According to an another advantageous embodiment of the invention, it ispossible to reduce the polarization resistance of the electrode byplacing the metal:ceramic ratio in the electrode (in vol.-%) betweenapproximately 65:35 and approximately 85:15 and even more preferablybetween 70:30 and 80:20, i.e., by increasing the metal:ceramic ratio.Apparently, when the ratio is placed into the range according to thepresent invention, the favorable effect of a high metallic component fora low polarization resistance and the effect of a low porosityincreasing the polarization resistance (due to the relatively lowceramic component) are balanced in such a way that the polarizationresistance decreases in relation to the standard ratio (60:40).

It is in fact known from the aforementioned German Patent Application 4240 267 A1 that the metal component in Pt cermet electrodes is generallybetween 50 and 80 vol.-%. It is also known from German PatentApplication DE 42 40 267 A1 that it is possible to use other metallicoxides such as CeO₂ instead of or in addition to ZrO₂ as ion conductorsin the solid electrolyte. However, the use of the cited ranges of themetal:ceramic ratio and/or the ZrO₂:CeO₂ ratio is not known, and it isin particular not known and not suggested by the related art that it ispossible to reduce the polarization resistance while maintaining thecited ranges.

It is favorable if the CeO₂ is stabilized using an oxide from the groupY₂O₃, Gd₂O₃, Sc₂03 and HfO₂.

The electrodes of the present invention may be used advantageously ingas sensors for pumping off O₂ from test gases containing O₂.

Additional advantageous embodiments of the platinum metal cermetelectrodes may be found in the dependent claims.

THE DRAWING

The present invention is explained in detail below with reference toexemplary embodiments illustrated by the drawing in which

FIG. 1 shows a system for illustrating the invention in a schematiccross-sectional view,

FIG. 2 shows a diagram in which O₂ reduction flows are plotted againstthe applied voltage for different concentrations of Y-stabilized CeO₂ inthe ceramic matrix according to an aspect of the present invention and

FIG. 3 shows a diagram in which O₂ reduction flows are plotted againstthe applied voltage for different metal:ceramic ratios according to anaspect of the present invention.

The invention will be described below primarily using the example of Ptcermet electrodes. However, it should be made clear that the inventionmay be used to particular advantage in conjunction with such Pt cermetelectrodes and may be explained very clearly with them; however,numerous deviations from this example are possible within the context ofthe claims.

System 1 shown in FIG. 1 contains a thin substrate 2 made of a highlygastight solid electrolyte material having oxygen ion conductivity suchas Y-stabilized ZrO₂, diametrically opposed Pt cermet electrodes 3 and 4having been applied to its two surfaces. The two electrodes areelectrically connected to a DC voltage source 5 in such a way thatelectrode 3 is the cathode and electrode 4 is the anode. An ammeter 6 isinstalled in one of the feeds to determine the current.

As an alternative, the metallic component in the electrodes may be madeup of another precious metal from the eighth subgroup of the periodicsystem of the elements or of mixtures of such elements such as Pt/Rh,Pt/Ir or Pt/Pd. In order to demonstrate the invention, differentproportions of the Y-stabilized ZrO₂ were replaced by stabilized CeO₂ atleast in the electrode connected as the cathode in a series of tests andthe metal:ceramic ratio was varied in another series of tests.

If a gas stream containing O₂ is directed past the heated system shownin FIG. 1, O₂ is electrochemically reduced at the electrode connected asthe cathode when the electrodes are energized and a corresponding O²⁻stream flows between the electrodes. One measure of the reductionactivity of the electrode is the polarization resistance (Rp). The Rp isa variable calculated from the initial slope of the cathodic I/Ucharacteristic, which is a measure of the resistance that the electrodeputs up to the O₂ reduction. The Rp should be as low as possible.

The inventors have determined that the polarization resistence isreduced in relation to that determined for standard electrodes if aspecific quantity of the Y-stabilized ZrO₂ is replaced by Y-stabilizedCeO₂ corresponding to the formula Ce_(0.84)Y_(0.16)O_(1.92). Thecathodic I/U characteristics in FIG. 2 show that if approximately 0.1wt.-% Y-stabilized ZrO₂ is replaced by Y-stabilized CeO₂ (molar ratio ofY-stab. ZrO₂:Y-stab. CeO₂=0.999:0.0008), the polarization resistance islower than if 1 or 5 wt.-% Y-stabilized ZrO₂ is replaced by Y-stabilizedCeO₂ (molar ratio of Y-stab. ZrO₂:Y-stab. CeO₂=0.99:0.0084 or Y-stab.ZrO₂:Y-stab. CeO₂=0.959:0.0405) or if the ZrO₂ contains no CeO₂. Thecharacteristics are based on measured data that was obtained on system 1(see FIG. 1) heated to approximately 700° C. with a test gas containingN₂ and 500 ppm O₂ flowing past.

Similar results are obtainable if the CeO₂ is stabilized using an oxidefrom the group Sc₂O₃, Gd₂O₃ and HfO₂ instead of with Y₂O₃. If Gd₂O₃ isused, the composition of the stabilized CeO₂ advantageously correspondsto the formula Ce_(0.8)Gd_(0.2)O_(1.9).

The inventors were also able to show that it is also possible toinfluence the polarization resistance by varying the metal:ceramic ratioin the electrodes (at least in electrode 3 connected as a cathode). Thecathodic I/U characteristics in FIG. 3 substantiate that thepolarization resistance is lower if the metal:ceramic ratio is in therange between approximately 70:30 arid approximately 80:20 than if theratio corresponds to the standard value (60:40) or is clearly above80:20. The characteristics are based on measured data which was obtainedon system 1 heated to approximately 700° C. (see FIG. 1) with a test gascontaining N₂ and 500 ppm O₂ flowing past.

The calculated polarization resistances were at the values indicated inthe table:

TABLE Metal:ceramic ratio (vol.-%) Rp (Ω) 60:40 83.7 (standard) 70:3055.3 80:20 50.6 90:10 221.5

In order to produce standard Pt cermet electrodes applied to a solidelectrolyte substrate, an initial stage of the solid electrolyte bodyformed from ZrO₂, stabilized Y₂O₃ and customary additives and asuspension are prepared, which in a proportion by volume of 60:40contains Pt powder and a correspondingly fine-particle preformed ceramicprecursor of a raw material mixture of fixed weight quantities of ZrO₂and Y₂O₃ and if necessary a small quantity of—for example—Al₂O₃, theceramic components being mixed into a paste with the Pt powder. Theunsintered solid electrolyte body is then impressed with the paste at alayer thickness of 10 to 30 μm followed by drying and finally the pasteand the solid electrolyte substrate are sintered together into a solidat between approximately 1300 and approximately 1600° C.

If electrodes are to be produced that differ from the standardelectrodes by only a greater metal:ceramic ratio, apart from usingrelatively larger Pt powder quantities, it is possible to proceed justas described above.

If electrodes are to be produced, with one part of the Y-stabilized ZrO₂being replaced by Y- or Gd-stabilized CeO₂, a Ce salt and a Y salt or aGd salt are precipitated together in the correct proportion, dried,calcined and the fixed quantity is then added to the raw materialmixture containing Y₂O₃ and ZrO₂. Apart from these additional processingsteps, the production method is carried out as described above.

System 1 shown in FIG. 1 is advantageously used in a gas sensor toanalyze gas mixtures containing O₂ as part of a pump cell for pumpingoff O₂ from the test gases, at least the cathode being designedaccording to the present invention.

1. A platinum metal cermet electrode for an electrochemical reduction ofoxygen, comprising: a ceramic component including stabilized ZrO₂,wherein: the cermet electrode includes a metal:ceramic ratio (in vol.-%)between 70:30 and 80:20 to reduce a polarization; and betweenapproximately 0.01 and approximately 1 wt.-% of the stabilized ZrO₂ isreplaced by stabilized CeO₂.
 2. The cermet electrode as recited in claim1, wherein: between approximately 0.02 and approximately 0.1 wt.-% ofstabilized ZrO₂ is replaced by stabilized CeO₂.
 3. The cermet electrodeas recited in claim 2, wherein: the stabilized CeO₂ is stabilized usingan oxide from the group Y₂O₃, Gd₂ O₃, Sc₂ O₃ and HfO₂.
 4. The cermetelectrode as recited in claim 2, wherein: a composition of thestabilized CeO₂ is stabilized using an oxide of a trivalent metal thatroughly corresponds to the formula Ce_(0.8)Me′″_(0.2)O_(1.9).
 5. Thecermet electrode as recited in claim 4, wherein: the composition of thestabilized CeO₂ is stabilized using Gd₂O₃ corresponding to the formulaCe_(0.8)Gd_(0.2)O_(1.9).
 6. The cermet electrode as recited in claim 4,wherein: the composition of the stabilized CeO₂ is stabilized using Y₂O₃corresponding to the formula Ce_(0.775)Y_(0.225)O_(1.88).
 7. The cermetelectrode as recited in claim 1, wherein: the ZrO₂ is stabilized usingY₂O₃.
 8. The cermet electrode as recited in claim 7, wherein:approximately 8 to approximately 15 wt.-% Y₂O₃ is present with respectto a quantity of the ZrO₂.
 9. The cermet electrode as recited in claim1, further comprising: a structure including a platinum metal thatincludes at least one metal from the group Pt, Rh, Ir, and Pd.
 10. Thecermet electrode as recited in claim 9, wherein: a metallic component ofthe cermet electrode is selected from one of a metal and a metal alloyfrom the group Pt, Pt/Rh, Pt/lr and Pt/Pd.
 11. An O₂-reducing systemconnectable to a voltage source, comprising: a solid electrolytesubstrate including stabilized ZrO₂ and a plurality of platinum metalcermet electrodes applied to both sides of the solid electrolytesubstrate, each cermet electrode including: a ceramic componentincluding stabilized ZrO₂, wherein the cermet electrode includes ametal:ceramic ratio (in vol.-%) between 70:30 and 80:20 to reduce apolarization resistance, and between approximately 0.01 andapproximately 1 wt.-% of the stabilized ZrO₂ is replaced by stabilizedCeO₂.
 12. The O₂-reducing system as recited in claim 11, wherein: atleast one cermet electrode forms a cathode.